Endogenous metabolism by sperm in response to altered cellular ATP requirements.

A quantitative description of the relative importance of endogenous metabolism to overall ATP production has not been established for mammalian cells. We report herein results of experiments using sperm (selected because of their simple metabolic potential and absence of biosynthetic pathways) and calorimetry (chosen because it serves as a general monitor of metabolism) to assess the importance of endogenous metabolism to total ATP synthesis under several incubation conditions. In experiments in which sperm were incubated at different temperatures (20 degrees C, 25 degrees C, 30 degrees C or 35 degrees C) and with different substrates (glucose, fructose, lactate, or beta-hydroxybutyrate), endogenous metabolism occurred at a constant rate regardless of the rate of ATP turnover in the cells or the nature of the exogenous substrate available to them. Sperm incubated at 35 degrees C with glycolyzable substrates synthesized more ATP (9 mumol ATP . h-1/10(8) cells) than did sperm incubated with the nonglycolyzable substrate, lactate (6.2 mumol ATP . h-1/10(8) cells). To investigate this substrate-related difference in the rate of ATP synthesis, the motility of sperm incubated at 35 degrees C with glucose or with lactate was determined. The velocities of the sperm incubated with either substrate were identical, indicating that the rates of ATP consumption for support of motility were identical. Most of the additional ATP synthesized by cells with glycolyzable substrates was consumed in the process of substrate cycling of the metabolic intermediates of glucose.